Steel pipes or tubing which are intended for underground installation must be protectively coated against corrosion. There is a substantial body of art relating to apparatus and methods for the application of a thermoplastic covering to a pipe, first covered with an adhesive coating.
"Cross-head" extrusion will be understood throughout the description and claims herein to refer generally to known processes for coating a pipe by conveying it longitudinally through the annular nozzle or head of an extruder, operable to extrude tubular coatings of adhesive film and jacketing material over the pipe as it passes through the extrusion head. Examples of cross-head extrusion apparatus and of the use of such in jacketing a steel pipe are referred to in U.S. Pat. Nos. 4,386,996 (Landgraf et al.); 4,484,877 (Stucke); 4,451,413 (Stucke et al.); 4,510,007 (Stucke); and 4,519,863 (Landgraf et al.).
The process of cross-head extrusion is variously referred to as a "straight-through" or "endo" process to distinguish it from the older known technique of enveloping a steel pipe by spiral wrapping. That method involves the simultaneous winding of pipe with a double layer of sheet material, the inner being an adhesive strip and the outer thermoplastic, in screw thread fashion as disclosed, for example, in U.S. Pat. No. 3,616,006 (Landgraf et al.).
Where it can be used, the "endo" process for applying an adhesive-thermoplastic multilayer covering to pipe generally produces a product having properties superior to a pipe spiral-wrapped with like materials. Known pipe coatings applied using a spiral method are subject to weak joints at the overlap and poor coverage of radial or longitudinal welding seams on the pipe. We have found that the disadvantages of spiral wrapping are particularly great where HDPE is to be applied as the outer jacketing material. Pipe which has been spiral-wrapped with that jacketing material often exhibits relatively poor low temperature adhesion of the protective coating.
A number of methods for the jacketing of steel pipe by the application of an adhesive-polyethylene laminate to a pipe which has first been coated with a curable epoxy resin/curing agent mixture (which mixtures will be referred to hereinafter generally as "epoxy primer") have been described in German printed Patent No. 19 65 802 (spiral wrapping method) and in U.S. Pat. No. 4,510,007 (Stucke) (straight-through extrusion). However, the "endo" process presents its own practical difficulties in the effective jacketing of pipe using such materials, particularly when high-density polyethylene is used as the outer cladding.
Firstly, in the "endo" process the application of an even layer of epoxy primer, critical to good results, is not a straightforward matter, unlike in the spiral process, where liquid primer can be applied to the surface of a pipe turned about its long axis. It has been proposed, in U.S. Pat. No. 4,510,007 referred to above, to apply primer electrostatically as a powdery precondensated blend of epoxy resin and curing agent. However, this requires supplemental surface heating of the pipe to melt the powder, and very careful temperature control to evaporate reaction products from the epoxy cure and obtain a reasonably uniform layer of primer base coating. According to the present invention, a novel annular epoxy primer applicator is employed, which can be used with a liquid primer such as a "100% solids" composition, for excellent control of the thickness of the primer base coat.
Secondly, the application of sealing pressure to the freshly-applied jacketing presents considerable difficulties when cross-head extrusion is used to coat the pipe. A pressure roller of the kind useful in applying thermoplastic strip helically, as in U.S. Pat. No. 4,178,200, cannot be used on the hot tubular jacket of thermoplastic or adhesive/thermoplastic multilayer composite deposited on a pipe coming from a cross-head extruder.